Apparatus for measuring magnetic field of microwave-assisted head

ABSTRACT

A measuring circuit system in a magnetic field measuring apparatus of the invention has an amplifier and a band-pass filter connected in sequence on an output terminal side of the TMR element, the band-pass filter is a narrow-range band-pass filter such that a peak pass frequency of the filter that is a center is a basic frequency selected from a range of 10 to 40 GHz and a band width centered around the basic frequency is a narrow range of ±0.5 to ±4 GHz; and with the measuring circuit system, an SIN ratio (SNR) of 3 dB or greater is obtained, the SNR being defined by a ratio of an amplitude S of a high-frequency generated signal induced by the TMR element to a total noise N that is a sum of a head noise generated by the TMR element and a circuit noise generated by the amplifier. With such a configuration, an in-plane high-frequency magnetic field generated by a microwave-assisted magnetic head is reliably and precisely measured.

BACKGROUND OF THE INVENTION TECHNICAL FIELD

The present invention relates to a device for measuring the in-planehigh-frequency magnetic field generated by a microwave-assisted magnetichead.

BACKGROUND

In the past, surface recording density has been notably increased inmagnetic recording media, such as hard disks or the like, byimprovements, for example, of finer magnetic particles that configure amagnetic recording layer, of materials and finer head processing.Furthermore, recently magnetic recording and reproducing apparatuseshave been commercialized with perpendicular magnetic recording methodsin which the surface recording density is further improved bymagnetizing the recording layer in a direction perpendicular to thesurface of the magnetic recording medium. Further improvements in thesurface recording density are anticipated in the future.

On the other hand, it is preferable to use as recording materialsmagnetic particles with large magnetic anisotropic energy Ku (magneticanisotropic magnetic field Hk) and large coercive force He becausethermal fluctuation of the recording magnetization tends to occur incorrespondence to the greater fineness of recording bits and magneticparticles.

However, when magnetic particles with large magnetic anisotropic energyKu are used as recording layer materials, the coercive force He of therecording layer becomes a large value of, for example, 4 kOe or more.When accomplishing saturation magnetic recording, it is generally saidthat a recording magnetic field of at least twice as large as thecoercive force is necessary. Therefore, with the performance ofconventional magnetic heads, cases arose in which it was extremelydifficult to achieve saturation magnetization of the recording layer. Inother words, there were cases in which recording and erasing magneticdata were difficult.

Magnetic recording of data onto a magnetic recording medium isaccomplished by a perpendicular recording magnetic field generated fromthe tip of the main magnetic pole of the magnetic head. Theperpendicular recording magnetic field is generated by applying acurrent to a main coil positioned adjacent to the main magnetic pole.One method has been studied in order to greatly reduce the perpendicularrecording magnetic field that is required for causing the magnetizationreversal. The method is conducted by overlapping alternating magneticfields in an in-plane direction in the microwave band equal to or closeto the ferromagnetic resonant frequency of the medium recording layer toa perpendicular recording magnetic field that induces such amagnetization reversal. This assisted recording method is known asmicrowave assisted magnetic recording (MAMR), and its efficacy has beenverified experimentally.

Two methods have primarily been proposed for MAMR. One is a method inwhich a spin torque oscillator (STO) made of multiple layers of magneticthin film is formed in a gap between the main magnetic pole and theauxiliary magnetic pole of the magnetic head and a microwave magneticfield in the in-plane direction is generated by driving a bias currentand causing the STO to oscillate, for example as noted in referencedocument 1 (J. Zhu et al; IEEE TRANSACTION ON MAGNETICS, Vol. 44, No. 1,p. 125) (this is sometimes called a self-excited type).

The other is a method in which a supplementary coil is prepared inand/or in the vicinity of the gap between the main magnetic pole and theauxiliary magnetic pole of the magnetic head, and an in-planealternating magnetic field is generated by driving the alternatingcurrent of the microwave band in the supplementary coil, for example asnoted in reference document 2 (Japanese Laid-Open ApplicationPublication No. 2007-299460) (sometimes called an induced type).

When considering mass production and commercialization of this type ofmagnetic head, the high frequency in-plane magnetic field intensitiesproduced by the microwave-assisted magnetic heads must each be preciselymeasured in order to secure the reliability of the device. Therefore, itis necessary to develop a highly sensitive, low-cost property measuringapparatus.

However, the following significant technical issues are faced indeveloping this device.

-   (1) In both self-excited and induced types, a gap between the main    magnetic pole and the auxiliary magnetic pole, and another gap in    the vicinity thereof, where the in-plane high-frequency magnetic    field is generated, are both assumed to be around 30 nm at the    largest, so the in-plane high-frequency magnetic field is generated    from an extremely tiny region.-   (2) In order to realize the microwave-assisted effect, it is    considered that a large in-plane high-frequency magnetic field is    necessary, for example 2 kOe or greater. Such an in-plane    high-frequency magnetic field is required.-   (3) The frequency of the in-plane high-frequency magnetic field is    the same as or close to the ferromagnetic resonant frequency of the    recording layer of the magnetic recording medium that is the target    of recording. The value is high because it is said that the value is    generally in the range of 10 GHz to 40 GHz.

On the other hand, as a method for measuring the recording magneticfield of magnetic heads used in conventional longitudinal recording, amethod has been proposed in which magnetic sensors, specifically a giantmagnetoresistive (GMR) heads, are positioned opposite to a flyingsurface of the magnetic head (see Japanese Laid-Open ApplicationPublication No. 2009-301610).

However, in the details of the proposal in the Japanese publication, thefrequency of the recording drive current is around 20 to 700 MHz. Thisfrequency is in a completely different frequency band from the 10 GHz to40 GHz frequency that the microwave-assisted magnetic head shouldmanifest.

In addition, when a GMR head is used as a measurement sensor, elementresistance is low and output is small, so reliable measurements areextremely difficult to obtain only by adjacent positioning because theSIN ratio of the measured signal cannot be guaranteed.

On the other hand, when a tunneling magnetoresistive (TMR) head havingmuch higher generated output than the GMR head is used as a measurementsensor, it is considered that reliable measurement is possible becausethe element resistance and its output are high so that the SIN ratio ofthe measurement signal can be guaranteed. However, with a TMR element,the intensity of the external magnetic field that can linearly respond(make a linear response) is at most several tens of Oe. Therefore,further engineering is necessary for measuring in-plane high-frequencymagnetic fields generated by microwave-assisted magnetic heads, thein-plane high-frequency magnetic fields being considered around 2 kOe ormore. Furthermore, in a TMR element, there is a difficulty to obtain thedesired S/N ratio because high-frequency characteristics are poor sincethe impedance with respect to element resistance is large, andresponsiveness corresponding to frequencies of 10 GHz or more generatedby the microwave-assisted magnetic head is inadequate.

The present invention was invented in light of these actualcircumstances, and it is an objective of this invention to provide ameasuring apparatus that can reliably and precisely measure the in-planehigh-frequency magnetic field generated by a microwave-assisted magnetichead.

Such a measuring apparatus can assure high density recording andimproved recording quality, and can contribute to simplifying, reducingthe cost of and increasing the throughput of shipping inspections.

SUMMARY OF THE INVENTION

In order to resolve the issues discussed above, the present invention isa magnetic field measuring apparatus for measuring an in-planehigh-frequency magnetic field intensity that is generated from amicrowave generation mechanism equipped with a microwave-assistedmagnetic head, the measuring apparatus including an anchoring mechanismfor anchoring the microwave-assisted magnetic head that is a target ofmeasurement, a high-frequency current driving system for applying ahigh-frequency current to the microwave generation mechanism equippedwith the microwave-assisted magnetic head, a magnetic sensor having atunneling magnetoresistive (TMR) element for measuring the in-planehigh-frequency magnetic field intensity generated from the microwavegeneration mechanism, a measuring circuit system connected to themagnetic sensor; and a stage capable of moving in three dimensionaldirections on which the magnetic sensor is mounted, wherein themeasuring circuit system has an amplifier and a band-pass filterconnected in sequence on an output terminal side of the TMR element, theband-pass filter is a narrow-range band-pass filter such that a peakpass frequency of the filter that is a center is a basic frequencyselected from a range of 10 to 40 GHz and a band width centered aroundthe basic frequency is a narrow range of ±0.5 to ±4 GHz; and with themeasuring circuit system, an S/N ratio (SNR) of 3 dB or greater isobtained, the SNR being defined by a ratio of an amplitude S of ahigh-frequency generated signal induced by the TMR element to a totalnoise N that is a sum of a head noise generated by the TMR element and acircuit noise generated by the amplifier.

In addition, as a preferred configuration of the magnetic fieldmeasuring apparatus of the present invention, the amplifier in themeasuring circuit system is configured with a pre-amplifier and a mainamplifier connected in sequence.

In addition, as a preferred configuration of the magnetic fieldmeasuring apparatus of the present invention, the amplifier in themeasuring circuit system is a main amplifier.

In addition, as a preferred configuration of the magnetic fieldmeasuring apparatus of the present invention, a comparator is connectednext to the narrow range band-pass filter in the measuring circuitsystem.

In addition, as a preferred configuration of the magnetic fieldmeasuring apparatus of the present invention, a low-pass filter and acomparator are connected in sequence next to the narrow-range band-passfilter in the measuring circuit system.

In addition, as a preferred configuration of the magnetic fieldmeasuring apparatus of the present invention, the magnetic sensormounted on the stage is positioned facing an air bearing surface (ABS)of the microwave-assisted magnetic head anchored by the anchoringmechanism, and the stage is moved so that the magnetic sensor scans apredetermined region of the ABS, and that the in-plane high-frequencymagnetic field intensity generated by the microwave generation mechanismequipped with the microwave-assisted magnetic head is measured by thismagnetic sensor.

In addition, as a preferred configuration of the magnetic fieldmeasuring apparatus of the present invention, the TMR element has alamination layer structure with a barrier layer interposed between twomagnetic layers.

In addition, as a preferred configuration of the magnetic fieldmeasuring apparatus of the present invention, the microwave-assistedmagnetic head that is the target of measurement provides a main magneticpole and an auxiliary magnetic pole, and a main coil for generating aperpendicular recording magnetic field is provided with the mainmagnetic pole, and a supplementary coil for driving an alternatingcurrent in the microwave band is provided in a gap formed with the mainmagnetic pole and the auxiliary magnetic pole and/or in the vicinitythereof.

In addition, as a preferred configuration of the magnetic fieldmeasuring apparatus of the present invention, an in-plane alternatingmagnetic field is generated by driving the alternating current in themicrowave band to the supplementary coil of the microwave-assistedmagnetic head, and a frequency of the alternating current issubstantially the same as a ferromagnetic resonant frequency of arecording layer of a magnetic recording medium that is a target ofrecording, and the frequency of the alternating current is in a range of10 GHz to 40 GHz.

In addition, as a preferred configuration of the magnetic fieldmeasuring apparatus of the present invention, the microwave-assistedmagnetic head that is the target of measurement provides a main magneticpole and an auxiliary magnetic pole, and a main coil for generating aperpendicular recording magnetic field is provided with the mainmagnetic pole, a spin torque oscillator is provided in a gap formed withthe main magnetic pole and the auxiliary magnetic pole and/or in thevicinity thereof, the spin torque oscillator being configured withmultiple magnetic thin films for generating an alternating current inthe microwave band.

In addition, as a preferred configuration of the magnetic fieldmeasuring apparatus of the present invention, a spin torque oscillatoris oscillated and an in-plane alternating magnetic field is generated bydriving a bias current to the spin torque oscillator of themicrowave-assisted magnetic head, and an oscillation frequency of thespin torque oscillator is substantially the same as a ferromagneticresonant frequency of a recording layer of a magnetic recording mediumthat is a target of recording, and the oscillation frequency is in arange of 10 GHz to 40 GHz.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a perspective view of the summary compositionof a magnetic field measuring apparatus for measuring the in-planehigh-frequency magnetic field generated by a microwave-assisted magnetichead.

FIG. 2 schematically shows the principle behind using a magnetic fieldmeasuring apparatus to measure the in-plane high-frequency magneticfield generated by a microwave-assisted magnetic head.

FIG. 3 is a graph showing one example of the effects of spacing whenmeasuring the in-plane high-frequency magnetic field.

FIG. 4 shows a basic composition of a measuring circuit system connectedto the magnetic sensor of a TMR element.

FIG. 5 shows a cross-section perpendicular to the track width directionof a magnetic recording medium and a thin film magnetic head that isconfigured with a supplementary coil between the main magnetic pole andthe auxiliary magnetic pole of the magnetic head and to generate thein-plane alternating magnetic field by driving (applying) an alternatingcurrent in the microwave band to the supplementary coil.

FIG. 6 is a cross-section taken along line II-II in FIG. 5.

FIG. 7 illustrates a medium opposing surface (or ABS) of the thin filmhead viewed from the direction of the arrow along line III-III in FIG.5.

FIG. 8 shows the schematic composition of a head gimbal assemblyincluding a supplementary coil driver, a variable capacitor and asuspension arm.

FIG. 9 is a schematic vertical cross-section in the track widthdirection of the magnetic recording medium and a thin-film magnetic headthat is a spin torque oscillator (STO) formed with multiple layers ofmagnetic thin film in a gap between the main magnetic pole and auxiliarymagnetic pole of the magnetic head.

FIG. 10 shows a comparison example illustrating the measuring circuitsystem connected to the magnetic sensor of the TMR element.

FIG. 11 is a graph illustrating the relationship between frequency andgain in the case of the comparison example of the measuring circuitsystem.

FIG. 12 is a graph showing the relationship between frequency and asignal/noise ratio (SNR) in the case of the comparison example of themeasuring circuit system.

FIG. 13 illustrates another preferred configuration of a thin-filmmagnetic head that provides a supplementary coil between the mainmagnetic pole and the auxiliary magnetic pole of the magnetic head andin which an alternating current in the microwave band is driven in thesupplementary coil, generating an in-plane alternating magnetic field.The figure is an enlarged view of the vicinity of the ABS and aschematic cross-sectional view perpendicular to the track widthdirection (reference number 14 indicates a magnetic recording mediumshown for reference so the positional relationship is clear).

FIG. 14 illustrates another preferred configuration of a thin-filmmagnetic head that provides a supplementary coil between the mainmagnetic pole and the auxiliary magnetic pole of the magnetic head andin which an alternating current in the microwave band is driven in thesupplementary coil, generating an in-plane alternating magnetic field.The figure is an enlarged view of the vicinity of the ABS and aschematic cross-sectional view perpendicular to the track widthdirection (reference number 14 indicates a magnetic recording mediumshown for reference so the positional relationship is clear).

FIG. 15 illustrates another preferred configuration of a thin-filmmagnetic head that provides a supplementary coil between the mainmagnetic pole and the auxiliary magnetic pole of the magnetic head andin which an alternating current in the microwave band is driven in thesupplementary coil, generating an in-plane alternating magnetic field.The figure is an enlarged view of the vicinity of the ABS and aschematic cross-sectional view perpendicular to the track widthdirection (reference number 14 indicates a magnetic recording mediumshown for reference so the positional relationship is clear).

FIG. 16 illustrates another preferred configuration of a thin-filmmagnetic head that provides a supplementary coil between the mainmagnetic pole and the auxiliary magnetic pole of the magnetic head andin which an alternating current in the microwave band is driven in thesupplementary coil, generating an in-plane alternating magnetic field.The figure is an enlarged view of the vicinity of the ABS and aschematic cross-sectional view perpendicular to the track widthdirection (reference number 14 indicates a magnetic recording mediumshown for reference so the positional relationship is clear).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

A magnetic field measuring apparatus of the present invention formeasuring the in-plane high-frequency magnetic field generated from amicrowave-assisted magnetic head is described with reference to FIG. 1to FIG. 4.

The in-plane high-frequency magnetic field generated from themicrowave-assisted magnetic head is, strictly speaking, generated from amicrowave generation mechanism contained in the microwave-assistedmagnetic head. As a microwave generation mechanism, two types are known:a self-excited type and an induced type. The structure of themicrowave-assisted magnetic head that is the target of thesemeasurements is described below.

FIG. 1 schematically shows a perspective view of the summary compositionof a magnetic field measuring apparatus for measuring the in-planehigh-frequency magnetic field generated by a microwave-assisted magnetichead. FIG. 2 schematically shows the principle behind using a magneticfield measuring apparatus to measure the in-plane high-frequencymagnetic field generated by a microwave-assisted magnetic head. FIG. 3illustrates a graph showing one example of the effects of spacing whenmeasuring the in-plane high-frequency magnetic field. FIG. 4 shoes thebasic composition of a measuring circuit system connected to themagnetic sensor of a TMR element.

As shown in FIG. 1, the magnetic field measuring apparatus according tothe present invention is provided with an anchoring mechanism 110 forholding and anchoring the microwave-assisted magnetic head 120 that isthe target of measurement, a high-frequency current driving system 130for applying a high-frequency current on the microwave generationmechanism 122 equipped with the microwave-assisted magnetic head 120, amagnetic sensor 150 having a TMR element for measuring the intensity ofthe in-plane high-frequency magnetic field 122 a generated from themicrowave generation mechanism 122, a measuring circuit system 160connected to the magnetic sensor 150, and a stage 155 capable of movingin the three dimensions of the X, Y and Z axes and on which the magneticsensor 150 is mounted.

A stand-type holding device shown in FIG. 1 is one example of theanchoring mechanism 110 for anchoring the microwave-assisted magnetichead, but there are no particular limitations on this mechanism. It isparticularly important for the structure of the anchoring mechanism 110to surely hold an object, while not holding the measured part, so thatthe in-plane high-frequency magnetic field 122 a can be easily measured,and the in-plane high-frequency magnetic field 122 a generated from themicrowave-assisted magnetic head 120 can be easily accomplished. Thisholding and anchoring includes a mechanism that can accomplishpositional anchoring through simply being placed. In addition, themicrowave-assisted magnetic head 120 that is the target of measurementmay be measured as the head portion alone, or may be measured in theassembled state as a so-called head gimbal assembly.

The microwave generation mechanism 122 is made of various parts providedwith a self-excited or an induced-type microwave generation element, asdescribed above.

In addition, the in-plane high-frequency magnetic field 122 a generatedfrom the microwave generation mechanism 122 is generated along the X-Zplane in FIG. 1.

The high-frequency current driving system 130 is provided with a systemthat can supply a predetermined high-frequency current so that thein-plane high-frequency magnetic field 122 a can be generated from themicrowave generation mechanism 122.

(Explanation of System for Measuring Intensity of In-PlaneHigh-Frequency Magnetic Field 122 a)

Next, the system for measuring the intensity of the in-planehigh-frequency magnetic field 122 a generated from the microwavegeneration mechanism 122 will be explained.

The intensity of the in-plane high-frequency magnetic field 122 a ismeasured by the magnetic sensor 150 positioned facing themicrowave-assisted magnetic head 120. The magnetic sensor 150 having aTMR element is an element having a so-called tunnel barrier film and astructure in which this tunnel barrier film is interposed between twoferromagnetic layers. The sensor 150 is similar to that used in thereading element in the thin-film head. The TMR element has large elementresistance and large output, and thus has the advantage that the SNratio is large, but conversely has the disadvantage that responsivenessto a high-frequency magnetic field is poor. Hence, in the presentinvention, the composition of the measuring circuit system 160 connectedto the magnetic sensor 150 is devised so as to improve responsiveness tothis high-frequency magnetic field. This is explained below.

The magnetic sensor 150, which contains the TMR element as a maincomponent, is mounted on the stage 155, which is capable to move in thethree dimensional directions of X, Y, and Z axes.

Adjusting the distance of the spacing of the microwave-assisted magnetichead 120 in the Z direction is an extremely important operation. Namely,it is necessary to weaken the in-plane high-frequency magnetic field ofaround 1 kOe or more generated by the microwave-assisted magnetic head120 to around 0.1 to 50 (Oe) in which the TMR element can linearlyrespond. In other words, the sensitivity must be controlled around 1/100(−40 dB). For that reason, it is necessary for the spacing Zs, which arethe gap distance in the Z direction, to be adjusted to around 0.3 to 30μm.

This is explained in greater detail with reference to FIG. 2 and FIG. 3.As noted above, FIG. 2 schematically shows the principle behind using amagnetic field measuring apparatus to measure the intensity of thein-plane high-frequency magnetic field 122 a generated by themicrowave-assisted magnetic head 120. In FIG. 2, the TMR element (150),which is a main component of the magnetic sensor 150 of the magneticfield measuring apparatus, is positioned opposite to the supplementarycoil 24 for driving the alternating current in the microwave band, whichis set in the gap between a main magnetic pole 20 and an auxiliarymagnetic pole 23. Both sides of the TMR element (150) are magneticallyshielded by shield members 151.

Calculation of the in-plane high-frequency magnetic field is testedtaking into consideration the effects of the spacing Zs. The gapmagnetic field Hg generated in FIG. 2 is 20,000 Oe (=2 T), and thehalf-gap G=15 nm. FIG. 3 shows the results of calculating thecorrelation between the spacing Zs (nm) and the received magnetic fieldintensity Hx (Oe). As shown in FIG. 3, by adjusting the spacing Zs, itis possible to arbitrarily control the received magnetic field intensityHx.

For reference, the data for received magnetic field intensity Hx (Oe)and spacing Zs (nm) are noted in Table 1 below.

TABLE 1 Zs (nm) Hx (Oe) 15 10000 30 5860 50 3960 75 2510 150 1260 300637 500 382 750 254 1500 127 3000 63 5000 38 7500 25 15000 13

When the TMR element is used as the sensor, the measurement can beperformed with a tolerance within a range of a few microns per meter.Therefore, this is an advantage that the adjustment of the closeness(managing the distance) becomes easy.

The distance in the Z direction can be determined with a certain margin(not necessary to fix the distance at the beginning), so it is fine toseek the magnetic field intensity profile through scanning in the X andY directions.

The three-dimensional movement mechanism in the X, Y and Z directionsmay, for example, accomplish minute movements in the X, Y and Zdirections through a micro-driving element using various types ofpiezoelectric bodies.

The magnetic sensor 150, which contains this type of TMR element as amain component, is linked to the measuring circuit system 160. Themeasuring circuit system 160 in the present invention has an amplifierand a band-pass filter connected in order on the output terminal side ofthe TMR element that is the magnetic sensor 150. Furthermore, themeasuring circuit system 160 is configured so that the SIN ratio (SNR)is 3 dB or greater, the S/N ratio being the ratio of the amplitude S ofthe high-frequency generated signal induced in the TMR element to thetotal noise N found by summing the head noise generated by the TMRelement and the circuit noise generated by the amplifier. Erroneousdetection may occur when the S/N ratio is small, the S/N ratio ispreferably 3 dB or greater.

The band-pass filter is a basic frequency of which the peak passingfrequency is selected from a range of 10 to 40 GHz. The peak passingfrequency is a center of the band. Further, the band-pass filter is anarrow band-pass filter of which the band width is in a range of ±0.5 to±4 GHz, preferably ±0.5 to ±2 GHz, that is centering the basicfrequency.

Theoretically, the narrower the band width is the more S/N ratioincreases. However when the band width is too narrow, the gain from thecentral frequency might decline, or waveform distortion might occur. Adesign of a band-pass filter that does not cause such problems isrequired.

The above-described basic frequency is selected from within 10 to 40 GHzin order to match the ferromagnetic resonant frequency of the magneticrecording film that is the target of recording and the frequency bandemitted by the microwave-assisted magnetic head that is the target ofmeasurements.

A preferred example of the measuring circuit system 160 is shown in FIG.4.

As shown in FIG. 4, the measuring circuit system 160 is linked to themagnetic sensor 150, and is configured by connecting a transfer circuit161, a pre-amplifier 162 that is one amplifier, a main amplifier 163that is one amplifier, a band-pass filter 164 and a low-pass filter 165in sequence. A comparator 170 is connected for determining good or badat the end.

In the magnetic sensor 150 surrounded by the dotted line in FIG. 4,reference character E indicates a high-frequency magnetic fielddetection voltage, reference character R_(H) indicates the resistance ofthe TMR element and reference character C_(H) indicates the capacitanceof the TMR element. Reference character R_(in) indicates the inputresistance of the pre-amplifier 162 and reference character C_(in)indicates the input capacitance of the pre-amplifier 162.

The low-pass filter 165 is not necessarily required, but providing thisoffers the advantage that it is possible to improve the S/N ratio forthe measuring system as a whole. It is preferred that the low-rangeblocking frequency is around 50 MHz and that the high-range blockingfrequency is set around 1.3 times larger than the ferromagnetic resonantfrequency of the recording film.

In the comparator 170, a comparison is performed of the standard voltageV_(S) and the regeneration voltage V_(R) measured at the final point ofthe filter of the measuring circuit system 160. When the regenerationvoltage Y_(R) is greater than the standard voltage V_(S), the product isdetermined to be good. As the terminus of the filter, the band-passfilter 164 is the terminus in a composition having only a band-passfilter 164, while in a compound composition with a band-pass filter 164and low-pass filter 165, the low-pass filter 165 is the terminus.

The measuring operation using the magnetic field measuring apparatus isperformed as described below.

The magnetic sensor 150 mounted on the stage 155 is positioned facingthe ABS (the so-called opposing surface of the magnetic recordingmedium) of the microwave-assisted magnetic head 120 anchored by theanchoring mechanism. Furthermore, by applying a high-frequency currenton the microwave generation mechanism 122 equipped with themicrowave-assisted magnetic head 120 from the high-frequency currentdriving system 130, the in-plane high-frequency magnetic field 122 a isgenerated from the microwave generation mechanism 122. Simultaneously,the stage is moved such that the magnetic sensor scans a predeterminedregion of the ABS. Thereby, the in-plane high-frequency magnetic fieldintensity from the microwave-assisted magnetic head is detected by themagnetic sensor, and the magnetic field intensity profile is obtained.

(Explanation of Microwave-Assisted Magnetic Head That Is Target ofMeasurement by Magnetic Field Measuring Apparatus of Present Invention)

Next, the structure of the microwave-assisted magnetic head that is thetarget of the in-plane high-frequency magnetic field intensity measuringapparatus of the present invention will be explained.

As noted above, microwave-assisted magnetic heads can be broadlyclassified into two types: self-excited and induced-type.

First, an ideal example of the induced-type will be described withreference to FIG. 5 through FIG. 8.

(Explanation of One Example of Induced-Type Microwave-Assisted MagneticHead)

As shown in FIG. 5, the thin-film magnetic head 12 has a recording head16 and a reproducing head 18. The magnetic recording medium 14 isprovided so as to face this thin-film magnetic head.

The recording head 16 is configured to have a main magnetic pole 20, aleading shield (auxiliary magnetic pole) 22 positioned on the front edgeof the recording side, a trailing shield (auxiliary magnetic pole) 23positioned on the trailing edge of the recording side, a main coil 24for generating a perpendicular recording magnetic field in the mainmagnetic pole 20 and a supplementary coil 26 for generating an in-planealternating magnetic field with frequencies in the microwave band in themain magnetic pole 20. The recording head 16 is configured such that themaximum value of the in-plane alternating magnetic field is smaller thanthe maximum value of the perpendicular recording magnetic field.

The perpendicular recording magnetic field is a magnetic field appliedin a substantially perpendicular direction (up and down on the surfaceof the paper) on the lamination layer surface of the recording layer 42of the magnetic recording medium 14. In addition, the in-planealternating magnetic field is a magnetic field applied in a directionsubstantially parallel to the surface of the lamination layer surface ofthe recording layer 42.

As shown in FIG. 6, the main magnetic pole 20 is configured such thatthe tip close to the magnetic recording medium 14 is narrower (tapered)than the base side when viewed from the leading shield 22 or thetrailing shield 23. In this drawing, the width of the tip is indicatedby Mpw. With such a configuration, it is possible to concentrate thewriting magnetic flux.

FIG. 7 shows a bottom view of the thin-film magnetic head as viewed fromthe magnetic recording medium 14 side, and shows a so-called ABS. Asshown in FIG. 7, the tip of the main magnetic pole 20 has a roughlytrapezoidal shape which is narrowing in the direction from the trailingshield 23 side to the leading shield 22 side. The maximum width of themain magnetic pole 20, indicated by the reference character Mpw, is thewidth of the surface facing the trailing shield 23 (the maximum width ofthe trapezoid).

Compared to the width Mpw of the tip of the main magnetic pole 20, thewidths of the leading shield 22 and the trailing shield 23 areexceptionally large. The leading shield 22 and the trailing shield 23are linked to the base of the main magnetic pole 20, although such isnot shown in the drawing.

In order to make the direction of the perpendicular recording magneticfield applied on the recording layer 42 approach the directionperpendicular to the surface of the magnetic recording medium 14, thegap between the main magnetic pole 20 and the leading shield 22 ispreferably 1 μm or more. In addition, the gap between the main magneticpole 20 and the trailing shield 23 is preferable 10 to 100 nm, and morepreferably around 50 nm.

The main coil 24 is positioned at the leading shield 22 side of the mainmagnetic pole 20 so as to enclose a linkage part (not shown) between themain magnetic pole 20 and the leading shield 22. In FIG. 5, the maincoil 24 is shown in the case of one winding but this is intended to beillustrative and not limiting. Two or more windings would also be fine.In addition, in FIG. 5 the main coil is single-layered, but this may bemulti-layered.

The supplementary coil 26 is positioned on the trailing shield 23 sideof the main magnetic pole 23 so as to enclose an linkage part (notshown) between the main magnetic pole 20 and the trailing shield 23. Asshown in this figure, a portion of the supplementary coil 26 ispositioned between the main magnetic pole 20 and the trailing shield 23.The thickness of the supplementary coil 26 is preferably 10 to 50 nm.

In general, the tip that is the surface of the main magnetic pole 20facing the magnetic recording medium is polished in the headmanufacturing process. Accordingly, in order to avoid the supplementarycoil 26 being polished, it is preferred to position (indent) the tip ofthe supplementary coil 26 around 10 nm or more behind from the tip ofthe main magnetic pole 20 in the separating direction from the magneticrecording medium 14.

The supplementary coil 26 is provided with an alternating magnetic fieldtransmission part 26A at a position opposite to the magnetic recordingmedium 14. Namely, the alternating magnetic field transmission part 26Ahas a shape that is virtually constant in the cross-sectionperpendicular to the direction of width, as shown in FIG. 5. Moreover,the alternating magnetic field transmission part 26A is shaped so as tobe roughly parallel in the direction of width and with a cross-sectionalarea that is smaller than the other parts of the supplementary coil 26,as shown in FIG. 6.

In addition, it is preferable for the width Sew of the alternatingmagnetic field transmission part 26A of the supplementary coil 26 to beset smaller than the width Mpw of the tip of the main magnetic pole 20,as shown in FIG. 6.

In FIG. 5, the supplementary coil 26 has a single winding, but thenumber of windings of the supplementary coil is not particularly limitedand may be two or more. In addition, in FIG. 5 the supplementary coilhas a single layer but is not particularly restricted to this and may bemulti-layered with two or more layers. By increasing the number ofwindings and number of layers of the supplementary coil 26, it ispossible to enlarge the in-plane alternating magnetic field whilecurtailing the current supplied to the supplementary coil 26.

The supplementary coil is exemplary shown in these drawings, which isonly positioned between the main magnetic pole 20 and the trailingshield 23, but it would also be fine to position multiple thick coilswith dimensions of several μm farther away from the ABS.

The coil may be single-winding or multiple-winding. With such a coil, itis possible to apply a large driving current and to increase a recordingmagnetomotive force (magnetomotive force, or motivated magnetic field).Concrete configurations of these are shown, for example, in FIG. 13 toFIG. 16, respectively. FIG. 13 illustrates another preferredconfiguration of a thin-film magnetic head that provides a supplementarycoil between the main magnetic pole and the auxiliary magnetic pole ofthe magnetic head and in which an alternating current in the microwaveband is driven in the supplementary coil, generating an in-planealternating magnetic field. The figure is an enlarged view of thevicinity of the ABS and a schematic cross-sectional view perpendicularto the track width direction (reference number 14 indicates a magneticrecording medium shown for reference so the positional relationship isclear). The tip shapes of the main magnetic pole 20 and the auxiliarymagnetic pole 23 of the magnetic head near the ABS are formed extremelynarrow (tapered), as illustrated in FIG. 13, and the supplementary coil126 arranged in this small gap is provided with a shape that is extendedto the rear side, which is the direction opposite the ABS, while the ABSside of the supplementary coil 126 is positioned in substantially thesame position as the tips of the main magnetic pole 20 and the auxiliarymagnetic pole 23. Furthermore, the length of the supplementary coil 126in the track width direction is the same length or longer than thelength of the tip of the main magnetic pole 20 in the track widthdirection. With this composition, it becomes possible to apply a largedriving current and to increase the magnetomotive force (motivatedmagnetic field).

The primary characteristic of the preferred configuration of thesupplementary coil 226 shown in FIG. 14 is that the supplementary coilshown in FIG. 13 is partitioned into three divided coils 226 a, 226 band 226 c, and these are mutually connected in parallel. Through this,it becomes possible to apply a large driving current and to increase themagnetomotive force (motivated magnetic field)

The primary characteristic of the preferred configuration of thesupplementary coil 326 shown in FIG. 15 is that one supplementary coil326 a is formed near the ABS side and furthermore a thickersupplementary coil 326 b is formed on the inner side (the side oppositethe ABS). Furthermore, the lengths of the supplementary coils 326 a and326 b in the track width direction are the same length or longer thanthe length of the tip of the main magnetic pole 20 in the track widthdirection. With this composition, it becomes possible to apply a largedriving current and to increase the magnetomotive force (motivatedmagnetic field).

The primary characteristic of the preferred configuration of thesupplementary coil 426 shown in FIG. 16 is that two thick supplementarycoils 426 a and 426 b are formed in order on the inner side from theABS. Furthermore, the lengths of the supplementary coils 426 a and 426 bin the track width direction are the same length or longer than thelength of the tip of the main magnetic pole 20 in the track widthdirection. With this composition, it becomes possible to apply a largedriving current and to increase the magnetomotive force (motivatedmagnetic field).

Each of the preferred configurations of the various supplementary coilsare described above. However, these are not necessarily limited to theseconfigurations.

As shown in FIG. 8, the thin-film magnetic head 12 is provided with asupplementary coil electric circuit 47 (including a supplementary coil26) in order to supply an alternating current to the supplementary coil26. The thin-film magnetic head 12 is provided with a suspension arm 48and a slider 50 attached to the tip thereof, and a recording head 16 anda reproducing head 18 are formed in the slider 50.

As shown in FIG. 8, the supplementary coil electric circuit 47 isprovided, near the base of the suspension arm 48, with a supplementarycoil driver 52 for supplying an alternating current with a frequency inthe microwave band in the range of 10 to 40 GHz to the supplementarycoil 26. A conductive layer 56, an insulating layer 58 and a conductivelayer 59 are layered, in this order, above the suspension arm 48 insequence. Furthermore, the supplementary coil driver 52 is electricallyconnected to the supplementary coil 26 via the conductive layer 56 andthe conductive layer 59, which form a planar waveguide.

The supplementary coil electric circuit 47 shown in FIG. 8 has avariable capacitor (impedance adjustment element) in the vicinity of theslider 50. It is preferred to design the sympathetic frequency of thesupplementary coil electric circuit 47 as close as possible to theferromagnetic resonant frequency of the recording layer 42 of themagnetic recording medium 14.

In FIG. 5, the reproducing head 18 has a leading shield 28, a magneticresistance effect element 30 and a trailing shield 32.

In FIG. 5, the magnetic recording medium 14 has, for example, asubstrate 34, an under layer 36, a soft magnetic layer 38, anorientation layer 40, a recording layer 42, an overcoat layer 44 and alubricating layer 46, layered in that order. The material of therecording layer 42 preferably has a perpendicular magnetic anisotropicenergy of 1×10⁶ erg/cc or greater. As specific materials for therecording layer 42, CoCrPt alloy may be used, for example. Theferromagnetic resonant frequency of the recording layer 42 is a uniquevalue for each material that is determined by the component elements andthe shape of the magnetic particles that configure the recording layer42, or the like. The magnetic recording medium 14 is attached to anunrepresented rotating mechanism so as to rotate in the directionindicated by the arrow in FIG. 5 (the right direction in FIG. 5).

The action of this type of thin-film magnetic head 12, and in particularthe recording head 16, is described below.

Namely, by supplying a direct current to the main coil 24 of therecording head 16, the main coil 24 generates a direct current magneticfield. This direct current magnetic field flows through the mainmagnetic pole 20, the soft magnetic layer 38 of the magnetic recordingmedium 14, the leading shield 22 and the trailing shield 23. Then, aperpendicular recording magnetic field is applied to the recording layer42 in the direction perpendicular to the surface of the magneticrecording medium 14.

On the other hand, when the supplementary coil electric circuit 47supplies an alternating current with a frequency in the microwave bandin the range of 10 to 40 GHz to the supplementary coil 26, thesupplementary coil 26 generates an alternating magnetic field. Thisalternating magnetic field is primarily transmitted to the main magneticpole 20 and the trailing shield 23 from the alternating magnetic fieldtransmission part 26A. Since this alternating magnetic field ishigh-frequency, due to the skin effect, the alternating magnetic fieldpenetrates the main magnetic pole 20 and the trailing shield 23.Particularly, the alternating magnetic field penetrates only limitedparts in the vicinity of the surface parts facing the alternatingmagnetic field transmission part 26A with respect to the main magneticpole 20 and the trailing shield 23.

In this manner, the alternating magnetic field generated by thesupplementary coil 26 is generated limited to the parts where the gapbetween the main magnetic pole 20 and the trailing shield 23 is smallbut reaches the deepest parts of the magnetic recording medium 14.Namely, the alternating magnetic field generated by the supplementarycoil 26 flows through the main magnetic pole 20, the recording layer 42and the trailing shield 23 so as to penetrate the recording layer 42 ina direction substantially parallel to the surface of the magneticrecording medium 14.

By applying the in-plane alternating magnetic field with a frequency inthe microwave band on the recording layer 42, it is possible to greatlyreduce the recording magnetic field in the perpendicular direction thatis needed for the perpendicular magnetic recording. For example,compared to the case of not applying the in-plane alternating magneticfield, by applying the in-plane alternating magnetic field it becomespossible to reduce the perpendicular magnetic field, which can reversethe magnetism of the recording layer 42 by 40% or more, and furthermore,reduction of up to around 60% is possible.

(Explanation of One Example of Self-Excited Type of Microwave-AssistedMagnetic Head)

Next, an ideal example of a self-excited microwave-assisted magnetichead will be simply explained with reference to FIG. 9.

FIG. 9 shows the components of self-excited microwave assisted recordingthat could be called the basic composition. This composition isdisclosed in the Reference Document: IEEE Transactions on Magnetics,Vol. 44, No. 1, January 2008, pp 125-131.

This self-excited microwave-assisted magnetic head is composed, in aregular recording head structure, by inserting a microwave generatingelement 200 having a structure similar to the MR element, such as a TMRelement or CPP-GMR element or the like, between the main magnetic poleand the trailing shield (same meaning as the auxiliary magnetic pole)thereof.

With regard to the magnetic recording medium 14, the configuration inabove-described FIG. 5 is simplified and only the soft magnetic layer38, the orientation layer 40 and the recording layer 42 are shown, butthe actual composition is the same as the configuration shown in FIG. 5.The composition of the recording head is shown simplified as well, butthe composition of the actual recording and shield cores are the same asthe configuration shown in FIG. 5.

As shown in FIG. 9, the microwave generating element 200 interposedbetween the main magnetic pole 20 and the auxiliary magnetic pole 23, asshown in detail in the enlarged diagram, has a reference layer 210 whosemagnetization is fixed in the perpendicular direction, a metallicinterlayer 220, a high saturation moment field generating layer 230 anda layer with perpendicular anisotropy 240 layered in the order.Electrodes 210 and 205 are positioned at the two sides of this layering.

The high saturation moment field generating layer 230 and the layer withperpendicular anisotropy 240 are magnetically exchange-coupled, formingan oscillating stack.

By applying a current for spin excitation from the reference layer 210side to the oscillating stack side, perpendicularly polarized spins aretransmitted to the oscillating stack (high saturation moment fieldgenerating layer 230) and the layer with perpendicular anisotropy 240from the reference layer 210 via the metallic interlayer 220. Due to aprecession movement of the oscillating stack, microwaves are generatedand a high-frequency electromagnetic field is emitted.

The high-frequency electromagnetic field that is generated has ahigh-frequency magnetic field component in the layer in-plane directionof the perpendicular magnetized layer, at the position of theperpendicular magnetization layer of the magnetic recording medium(magnetic disk). By exposing such a high-frequency electromagnetic fieldto a portion of the perpendicular magnetization layer, the anisotropicmagnetic field Hk of the exposed portion can be reduced. The anisotropicmagnetic field Hk is a physical quantity that determines the coerciveforce Hc. A writing magnetic field is applied from the tip of the mainmagnetic pole 20 on the part where the anisotropic magnetic field Hk wasreduced. With the process, it becomes possible to write to theperpendicular magnetized layer, which has an extremely large anisotropicmagnetic field Hk, making it possible to realize good and so-calledmicrowave-assisted magnetic recording.

It is preferred for the frequency of the high-frequency electromagneticfield generated from the oscillating stack that forms theabove-described microwave generation element to be substantially thesame as the magnetic resonant frequency of the magnetic recording layerof the magnetic recording medium that is the target of writing.

Embodiments

The invention of a device for measuring the in-plane high-frequencymagnetic field produced from the microwave-assisted magnetic headdescribed above will be explained in further detail through the concreteembodiments shown below.

(First Embodiment)

COMPARISON EXAMPLE

in the measuring circuit system 160 shown in FIG. 4, the main amplifier163, the band-pass filter 164 and the low-pass filter 165 were omitted.Furthermore, a measuring circuit system 160′ of a comparison example,such as that shown in FIG. 10, was formed.

Using the measuring circuit system 160′ of this type of comparisonexample, studies were conducted to determine whether or not it waspossible to ensure the S/N ratio (hereinafter abbreviated as simple SNR)of the generated signal output.

(Calculation of Generated Output of TMR Element, TMR Element Resistanceand Pre-amplifier 162 Resistance)

In calculating the SNR, important parameters are the generated output ofthe TMR element 150, the generated output prior to the dividedpre-amplifier 162, the TMR element resistance and the input resistanceof the pre-amplifier 162. Typical values for mass-produced products wereused as these values.

Namely, the TMR element resistance was 250Ω; the generated output of theTMR element was 20 mV; and the input resistance Rin of the pre-amplifier162 was 100Ω. Using these values, the generated output Vpp prior to thepre-amplifier 162 is calculated to be:Vpp=20×(100/(100+250))=5.7 mVThe SNR was measured using this generated output as the standard.

The cut-off frequency f₀ is expressed as:f ₀=(½π·c·R)Here, c=C_(H)+C_(in) and R=R_(in)·R_(H)/(R_(H)+R_(in)).

Assuming that the total capacitance was 2.5 pF, and when the cut-offfrequency f₀ was calculated using the above formula, the cut-offfrequency (3 dB down) was calculated around 1 GHz, roughly matching theexperimental data (1 to 1.5 GHz) shown in FIG. 11. Through this, thevalidity of the above analytical method was confirmed. FIG. 11 shows themeasured results for voltage gain properties versus frequency when theinput resistance Rin of the pre-amplifier 162 was 100Ω. The voltage gainwas displayed as:G=|V _(R) /E|In FIG. 11, the 3 dB cut-off frequency was around 1 GHz, virtuallymatching the above analysis. In the band above 1 GHz, attenuation of 20dB/dec occurred following the gain properties of the following equation.

${F(f)} = \frac{1}{\sqrt{\left( {1 + \left( {f/{fo}} \right)^{2}} \right.}}$

SNR (dB) versus frequency was calculated and the results are shown inFIG. 12.

From the results shown in FIG. 12, it can be seen that when thefrequency is 20 GHz, SNR≈0 dB, and in the measuring circuit system 160′of the comparison example, when the frequency is a high frequency of 20GHz or higher, discrimination of the signal is impossible so measuresfor improvement are necessary.

(Embodiment)

In place of the measuring circuit system 160′ (see FIG. 10) used in theabove-described comparison example, a measuring circuit system that issimilar to that shown in FIG. 4 was introduced. Namely, a main amplifier163, which is one amplifier, and a band-pass filter 164 were added nextto the pre-amplifier 162. In this embodiment, the low-pass filter 165was not used.

The main amplifier 163 used is configured to further amplify a signalthat is amplified by the pre-amplifier so that discrimination processingwith the comparator is possible.

Furthermore, when 20 GHz±1 GHz (band width 2 GHz) was used as theband-pass filter 164, the SNR was around 9.9 dB, so in the 20 GHz±1 GHzband, it was possible to achieve SNR=3 dB or greater.

In addition, when 30 GHz±1 GHz (hand width 2 GHz) was used, the SNR wasaround 6.4 dB, so in the 30 GHz±1 GHz band, it was possible to clearSNR=3 dB or greater.

The efficacy of the present invention is obvious from the aboveexperimental results.

The magnetic field measuring apparatus of the present invention has ananchoring mechanism for anchoring the microwave-assisted magnetic headthat is the target of measurement, a high-frequency current drive systemfor applying a high-frequency current on the microwave generationincluded in the microwave-assisted magnetic head, a magnetic sensorhaving a TMR element for measuring the in-plane high-frequency magneticfield intensity generated by the microwave generation, a measuringcircuit system connected to the magnetic sensor, and a stage providedcapable of moving in three dimensions on which the magnetic sensor ismounted, such that the measuring circuit system has an amplifier and aband-pass filter connected in order on the output terminal side of theTMR element, the band-pass filter is a narrow range band-pass filtersuch that the central peak pass frequency is a basic frequency selectedfrom within 10 to 40 GHz and the band width centered on this basicfrequency is ±0.5 to ±4 GHz, and through this measuring circuit system,an SIN ratio (SNR) of 3 dB or greater is obtained, this S/N ratio beingthe ratio of the amplitude S of the high-frequency generated signalinduced in the TMR element to the total noise N that is the sum of thehead noise generated by the TMR element and the circuit noise generatedby the amplifier. Consequently, this measuring apparatus can reliablyand precisely measure the in-plane high-frequency magnetic fieldproduced by a microwave-assisted magnetic head. Furthermore, such ameasuring apparatus can assure high density recording and improvedrecording quality, and can contribute to simplifying, reducing the costof and increasing the throughput of shipping inspections.

1. A magnetic field measuring apparatus for measuring an in-planehigh-frequency magnetic field intensity that is generated from amicrowave generation mechanism equipped with a microwave-assistedmagnetic head, the measuring apparatus comprising: an anchoringmechanism for anchoring the microwave-assisted magnetic head that is atarget of measurement; a high-frequency current driving system forapplying a high-frequency current to the microwave generation mechanismequipped with the microwave-assisted magnetic head; a magnetic sensorhaving a tunneling magnetoresistive (TMR) element for measuring thein-plane high-frequency magnetic field intensity generated from themicrowave generation mechanism; a measuring circuit system connected tothe magnetic sensor; and a stage capable of moving in three dimensionaldirections on which the magnetic sensor is mounted, wherein themeasuring circuit system has an amplifier and a band-pass filterconnected in sequence on an output terminal side of the TMR element, theband-pass filter is a narrow-range band-pass filter such that a peakpass frequency of the filter that is a center is a basic frequencyselected from a range of 10 to 40 GHz and a band width centered aroundthe basic frequency is a narrow range of ±0.5 to ±4 GHz; and with themeasuring circuit system, an SIN ratio (SNR) of 3 dB or greater isobtained, the SNR being defined by a ratio of an amplitude S of ahigh-frequency generated signal induced by the TMR element to a totalnoise N that is a sum of a head noise generated by the TMR element and acircuit noise generated by the amplifier.
 2. The magnetic fieldmeasuring apparatus of claim 1, wherein the amplifier in the measuringcircuit system is configured with a pre-amplifier and a main amplifierconnected in sequence.
 3. The magnetic field measuring apparatus ofclaim 1, wherein the amplifier in the measuring circuit system is a mainamplifier.
 4. The magnetic field measuring apparatus of claim 1, whereina comparator is connected next to the narrow range band-pass filter inthe measuring circuit system.
 5. The magnetic field measuring apparatusof claim 1, wherein a low-pass filter and a comparator are connected insequence next to the narrow-range band-pass filter in the measuringcircuit system.
 6. The magnetic field measuring apparatus of claim 1,wherein the magnetic sensor mounted on the stage is positioned facing anair bearing surface (ABS) of the microwave-assisted magnetic headanchored by the anchoring mechanism, and the stage is moved so that themagnetic sensor scans a predetermined region of the ABS, and that thein-plane high-frequency magnetic field intensity generated by themicrowave generation mechanism equipped with the microwave-assistedmagnetic head is measured by this magnetic sensor.
 7. The magnetic fieldmeasuring apparatus of claim 1, wherein the TMR element has a laminationlayer structure with a barrier layer interposed between two magneticlayers.
 8. The magnetic field measuring apparatus of claim 1, whereinthe microwave-assisted magnetic head that is the target of measurementprovides a main magnetic pole and an auxiliary magnetic pole, and a maincoil for generating a perpendicular recording magnetic field is providedwith the main magnetic pole, and a supplementary coil for driving analternating current in the microwave band is provided in a gap formedwith the main magnetic pole and the auxiliary magnetic pole and/or inthe vicinity thereof.
 9. The magnetic field measuring apparatus of claim8, wherein an in-plane alternating magnetic field is generated bydriving the alternating current in the microwave band to thesupplementary coil of the microwave-assisted magnetic head, and afrequency of the alternating current is substantially the same as aferromagnetic resonant frequency of a recording layer of a magneticrecording medium that is a target of recording, and the frequency of thealternating current is in a range of 10 GHz to 40 GHz.